Radio communication systems are widely spread all over the world to provide various types of communication services such as voice or data. In general, the radio communication system may be a multiple access system capable of supporting communication with multiple users by sharing available radio resources. Examples of the radio resource include a time resource, a frequency resource, a space resource, etc. Examples of the multiple access system include a time division multiple access (TDMA) system, a frequency division multiple access (FDMA) system, a spatial division multiple access (SDMA) system, etc. Time, frequency, and space resources are primary radio resources differently allocated to multiple users respectively to the TDMA system, the FDMA system, and the SDMA system.
In addition, the radio communication system is a system supporting bidirectional communication. The bidirectional communication can be performed by using a time division duplex (TDD) mode, a frequency division duplex (FDD) mode, etc. The TDD mode uses the time resource to identify uplink transmission and downlink transmission. The FDD mode uses the frequency resource to identify uplink transmission and downlink transmission.
The radio communication system includes a base station (BS) providing a service to a specific region (i.e., a cell). In general, a user equipment (UE) can communicate with the BS when the UE is located within a coverage of the BS. When the UE is located in a cell boundary or when an obstacle such as a building exists between the UE and the BS, communication quality between the UE and the BS may not be satisfactory.
Several methods are provided to extend the coverage of the BS. In one of the methods, the radio communication system employs a relay station (RS). For example, long term evolution (LTE)-advanced, which is a promising candidate technique of international mobile telecommunication (IMT)-advanced (i.e., a post 3rd generation mobile communication system), includes an RS technique among primary techniques.
The RS is an apparatus for relaying a signal between the BS and the UE, and is used to extend cell coverage of the radio communication system and to improve cell throughput. An uplink and a downlink between the BS and the RS are backhaul links. An uplink and a downlink between the BS and the UE or between the RS and the UE are access links. Hereinafter, a signal transmitted through the backhaul link is referred to as a backhaul signal, and a signal transmitted through the access link is referred to as an access signal.
It is difficult for the RS to transmit and receive a signal by using the same frequency band and the same time. For example, it is difficult for the RS to transmit an access signal while receiving a backhaul signal. This is because the access signal transmitted by the RS and the backhaul signal received by the RS act as interference to each other, which may result in signal distortion. This is called self interference (SI). In order for the RS to solve the SI problem, a complex SI cancellation process is required, and transmission and reception signal processors need to be separated spatially. In reality, it is difficult for the RS to cancel the SI, and even if it is implemented, great expenses are required.
The RS needs to report to UEs connected to the RS a subframe in which the backhaul signal is received from the BS. This is to prevent the UEs from performing an unnecessary signal reception operation since the RS cannot transmit the access signal in the subframe due to the SI. As one method of reporting the subframe by the RS to the UE, there is a method of configuring a subframe for receiving a backhaul signal as a multicast broadcast single frequency network (MBSFN) subframe.
The RS may report to the UE that a corresponding subframe is an MBSFN subframe by using a control signal transmitted in a duration of a first specific OFDM symbol and then may receive a backhaul signal in a duration of the remaining OFDM symbols. Such a method can be referred to as a relay method based on the MBSFN subframe.
As described above, although the MBSFN subframe can be used for receiving the backhaul signal by the RS from the BS, the MBSFN subframe is used in principle for a multimedia broadcast multicast service (MBMS). However, a method of identifying a usage of the MBSFN subframe and reporting the usage to the UE has not been considered in the conventional technique.
In addition, when one BS transmits a backhaul signal to two or more RSs in the relay method based on the MBSFN subframe, a multiplexing method for improving usage efficiency of radio resources is not taken into account.